1. Technical Field of the Invention
This invention relates to a composition comprised of a water soluble, particulate material confined in and surrounded by a coating which is permeable to water, but which is not water soluble. The invention also relates to a particulate reactive chemical confined in and surrounded by a coating which directly contacts the confined particulate reactive chemical. The invention further relates to a water soluble, particulate, reactive chemical confined in and surrounded by a coating comprised of an interior coating, which directly contacts the confined chemical, and an exterior coating. The invention particularly pertains to the chemical nature of the composition employed to produce the coating which directly contacts the particulate reactive chemical. The invention further pertains a method of making the composition. The invention still further pertains to the release of the particulate reactive chemical from the composition.
2. Description of the Prior Art and Problems Solved
The prior art has dealt with the problem of causing, or of creating the conditions to cause, a specified chemical to react in a specified environment or in a specified location at a specified time or over a specified period of time. This problem ordinarily stems from the need to initiate some chemical action in some desirable location prior to the start of, during or subsequent to the completion of some other action, and/or to cause the chemical action associated with a specified chemical to proceed over a desirable period of time. The related problems of controlling the time and place of reaction of a chemical, as well as the solutions thereto, have been variously referred to in the art as delayed release, continuous release and controlled release and are referred to as such herein.
The prior art has addressed the problems referred to above, and has developed various methods of controlling the introduction of an active chemical into reactive contact with an environment. Some solutions to the problems have involved preventing reactive contact of the active chemical with an environment followed by permitting reactive contact to occur at some later time. One of the methods developed features the steps of preventing the mentioned reactive contact by completely covering a small quantity of the active chemical with a material to confine the chemical in a capsule followed by introducing a number of such capsules into a designated environment wherein the chemical is released from the capsules to permit reactive contact.
The prior art has disclosed various different mechanisms for releasing an active chemical from a capsule into reactive contact with a designated environment. The disclosed release mechanisms have featured the use of capsules which are crushed to rapidly release the enclosed chemical; capsules which rupture, or burst, to rapidly release the enclosed chemical; capsules which dissolve or disintegrate to rapidly release the enclosed chemical; and capsules which do not rupture, but which rely on diffusion or permeation, to gradually release the enclosed chemical.
Chemicals known to have been released from a capsule by a controlled release mechanism have included medicines, pesticides, herbicides, cosmetics, laundry products, pigments, polymerization initiators, cross linking agents and viscosity reducing agents. A delayed and/or a continuous release of such chemicals from a confining material can offer the advantage of maximizing the effectiveness of the chemicals or of minimizing or eliminating undesirable chemical or physical results or both.
The differences between the various capsule-based controlled release methods of the prior art have resided in the nature of the material employed to form the coating or coatings on the capsule, and the specific mechanism of the controlled release. Examples of prior art methods involving a single coating are disclosed in various United States Patents including U.S. Pat. No. 4,756,844 (Walles I), U.S. Pat. No. 4,741,401 (Walles II), U.S. Pat. No. 4,923,753 (Walles III), U.S. Pat. No. 4,919,209 (King), and U.S. Pat. No. 5,373,901 (Norman et al).
Norman et al disclose a coating material comprising a partially hydrolyzed acrylic polymer crosslinked with either an aziridine prepolymer or a carbodiimide. The acrylic is defined as being comprised of a mixture of an unsaturated carboxylic acid and an alkyl acrylate or a methacrylate ester.
Reddy et al, in U.S. Pat. No. 6,444,316, disclose a dual coating system comprised of an interior coating in direct contact with the confined chemical, and an exterior coating in direct contact with the interior coating. The interior coating disclosed by Reddy et al is a dry hydrophobic product made from a material selected from the group consisting of styrene-butadiene rubber latex, waxes, oils, polybutylene and atactic polyolefins. According to Reddy et al, the interior coating may also be a sparingly soluble material. The exterior coating disclosed by Reddy et al is formed on the interior coating, and is a porous, cross-linked hydrophilic polymer comprised of partially hydrolyzed acrylic polymer which is cross-linked with either an aziridine prepolymer or a carbodiimide.
The principle difference between Reddy et al and Norman et al resides in the interior coating of Reddy et al. Norman et al and Reddy et al each disclose that their respective coating materials are applied to a particulate solid by a spray coating technique, referred to as a fluidized bed process. Thus, an aqueous solution of the coating material is sprayed on a particulate solid while the particulate is being fluidized by a hot gas such as air or nitrogen. The hot gas evaporates water from the aqueous solution leaving a dry porous membrane or film of the coating material on the particulate solid.
The patent art referred to above all disclose capsules containing reactive chemicals and the methods of releasing the chemicals from the capsules to enable the confined chemicals to react with a composition in contact with the exterior of the capsules. The disclosed chemicals range from those which treat water based liquids to those which treat oil based liquids. Walles I discloses an encapsulated bleach as a laundry detergent additive. The inventions of Walles II, King and Norman et al disclose an encapsulated breaker for a fracturing fluid in a subterranean formation. Walles III discloses an encapsulated acid. Reddy et al disclose encapsulated cement set time additives and strength accelerating agents, and encapsulated solvents for drilling fluid filter cake. Reddy et al disclose that their invention enables the successful encapsulation of calcium chloride, and other similar salts, acid and acid forming chemicals.
An important, if not the essential, feature of a method of controlling the release of a chemical composition from a capsule is the timing of the release. This feature can be illustrated in connection with the release of a chemical from a capsule to reduce the viscosity of a fracturing fluid subsequent to the performance of a fracturing treatment of a subterranean formation. Reducing the viscosity of a fracturing fluid is referred to in the art as “breaking” and the viscosity reducing chemical is referred to as a “breaker.” For reasons well known in the fracturing art, it is desirable that a fracturing fluid have a high viscosity during the performance of the fracturing process, but a low viscosity subsequent to the performance of the process. In this regard, a desired high viscosity is preferably to be observed in the vicinity of the subterranean formation to be treated and a desired low viscosity is also preferably to be observed in the vicinity of the subterranean formation, wherein the high viscosity condition occurs prior to and during the performance of the fracturing treatment and the low viscosity condition occurs subsequent to the performance of the fracturing treatment.
Breakers, upon reactive contact with the fracturing fluid, can effectively produce the desired reduction of viscosity of the fluid, but the timing of the break, that is, when the break occurs, is of critical importance. Breakers, upon reactive contact with the fracturing fluid, begin to reduce the viscosity of the fluid. Accordingly, the problem to be solved is how to intimately mix the breaker with the fracturing fluid to enable contact with the fluid, while at the same time delaying actual reactive contact. This problem has been addressed and solved by placing the breaker in a capsule, the wall of which is a coating which operates to shield the fracturing fluid from contacting the breaker. The capsule containing the breaker is then intimately mixed with the high viscosity fluid, and the coating, by one of the mentioned mechanisms, functions to release the breaker to enable reactive contact between the breaker and the fluid at some future time. The breaking of fracturing fluids and the use of encapsulated breakers to perform the task are subjects discussed in Norman et al.
An important property of the coating composition is its ability to resist a caustic environment, either acid or base, exhibited by the chemical enclosed in, and, thus, in contact with, the interior surface of the coating composition, and by the chemical in contact with the exterior surface of the coating composition. The sensitivity of the coating composition to a caustic environment is the subject of the disclosure of Walles III. A capsule constructed of a confining material which will function to hold and maintain diverse chemicals, such as, organic and inorganic caustics, salts and oxidizers, and which will also function to release the chemicals at some desirable time is desired by the art.